Analog-to-digital converters (ADCs) are commonly used in electronic devices, especially in the field of image sensing or image sensor. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) have been adopted and proliferated in many popular electronic devices; and the CIS is configured with ADCs.
Each of the ADCs may include a counter such as a ripple counter for outputting a counting value in a sampling operation of the ADC. However, the ripple counter's speed is limited to few hundred MHz because of huge peak, average current, and power/ground metal IR drop. As a resolution of image sensor increases, the counter's speed is expected to be higher to satisfy the market's demands. In addition, power consumption of the counter and the ADCs is another issue that need to be concerned.